The present invention relates to a proportional solenoid for driving a spool of a hydraulic control valve and also pertains to a hydraulic control valve using the solenoid.
FIG. 1 is a sectional view showing a structural example of a conventional solenoid of the type described above. The solenoid is a proportional solenoid that generates electromagnetic force proportional to the electric current supplied to an excitation coil. The solenoid 100 has a cylindrical casing 101. An axially movable plunger 102 is placed in the casing 101, together with an excitation coil 104 wound around a bobbin 103. The excitation coil 104 is disposed to surround the outer periphery of the plunger 102. The sides of the casing 101 are covered with covers 105 and 106.
A push pin 107 projects through the center of the cover 106 to transmit force generated from the plunger 102 and the displacement thereof to the outside of the solenoid 100. The cover 106 has a disk-shaped cover portion 106a made of a magnetic material and a cylindrical portion 106b projecting from the cover portion 106a in such a manner as to surround a part of the outer periphery of the plunger 102. The cylindrical portion 106b has a tapered portion at the distal end thereof. The tapered portion is engaged with a tapered portion of a non-magnetic cylindrical member 108. A magnetic cylindrical member 109 is engaged with an end of the non-magnetic cylindrical member 108 on the side thereof remote from the cover 106. The cylindrical portion 106b of the cover 106, together with the non-magnetic cylindrical member 108 and the magnetic cylindrical member 109, surrounds the plunger 102.
The tapered portion of the cylindrical portion 106b and the tapered portion of the non-magnetic cylindrical member 108 allow a part of the axial magnetic flux produced from the excitation coil 104 to escape to the outer peripheral side, whereby the axial attraction force acting on the plunger 102 is kept constant independently of the position of the plunger. The cover 106 is provided with a through-hole 111 communicating with a compartment 110 accommodating the plunger 102. The through-hole 111 is a hole for allowing fluid to come in and out of the compartment 110 therethrough in an amount corresponding to a change in the volume of fluid in the compartment 110 due to displacement of the plunger 102.
In FIG. 1, if the plunger 102 moves rightward from the solid-line position by dx to the broken-line position, an amount of fluid corresponding to APLdx flows in the space at the left-hand side of the plunger 102 from the right-hand space. Meanwhile, an amount of fluid corresponding to (APLxe2x88x92APIN)dx is discharged from the right-hand space. An amount of fluid corresponding to the volume difference APINdx is sucked into the solenoid 100 through the through-hole 111 from the outside of the solenoid 100. Here, APL denotes the sectional area of the plunger, and APIN denotes the sectional area of the push pin 107.
In the above-described conventional solenoid 100, the through-hole 111 formed in the cover 106 is at a position below the top of the cylindrical compartment 110 accommodating the plunger 102. Therefore, an air reservoir 112 is undesirably formed in the compartment 110. That is, in FIG. 1, the distance Dh from the axis of the plunger 102 to the uppermost part of the through-hole 111 is smaller than the distance Di from the axis of the plunger 102 to the uppermost part of the compartment 110. Consequently, the conventional solenoid 100 has a structure in which air stays in the upper part of the compartment 110 without being exhausted therefrom (i.e. the air reservoir 112 is formed).
In a case where the air reservoir 112 is not present, when the plunger 102 moves rightward in the figure, for example, the fluid at the right-hand side of the plunger 102 flows leftward, and at this time, a moderate damping action is applied to the plunger 102 by the viscosity of the fluid flowing from the right to the left. However, if there is air in the compartment 110, because the viscosity of the air is extremely smaller than that of a liquid used as a working fluid, the damping action applied to the plunger 102 is reduced, and hence vibrations occur unfavorably.
If the air reservoir 112 is present in the solenoid 100 as used in a hydraulic control valve having a damping orifice (described later), a change in the volume of the solenoid-side space due to the displacement of the spool is undesirably absorbed by the compressibility of the air. Consequently, the damping effect cannot be obtained, and hence the spool vibrates unfavorably. Accordingly, the operation of the hydraulic control valve cannot be stabilized.
Further, when water is used as a working fluid, if there is air in the compartment 110, the air oxidizes the plunger 102 and the surrounding members. This causes friction to increase and degrades performance unfavorably.
The present invention was made in view of the above-described circumstances. An object of the present invention is to provide a solenoid wherein air cannot be collected in the space inside the solenoid, and hence the plunger or the spool operates stably without vibrating, and there is no possibility of an increase in friction or performance degradation which would otherwise be caused by oxidation of the plunger and the surrounding members, and also provide a hydraulic control valve using the solenoid.
To solve the above-described problem, a first feature of the present invention resides in a solenoid having a cylindrical excitation coil and a plunger movable in the excitation coil and adapted to generate electromagnetic force to move the plunger when an electric current is supplied to the excitation coil. A cover for closing a side of a plunger compartment accommodating the plunger is provided with upper and lower through-holes extending through the cover from the outside of the solenoid to the plunger compartment. The uppermost part of the upper through-hole is above or level with the uppermost part of the plunger compartment. The lowermost part of the lower through-hole is below or level with the lowermost part of the plunger compartment.
A second feature of the present invention resides in a hydraulic control valve including a hydraulic control valve body having a spool sliding in a sleeve, and a solenoid having a plunger and an excitation coil for generating magnetic force to move the plunger. The solenoid is attached to the hydraulic control valve body to apply moving force to the spool by the movement of the plunger. A cover for a side of the solenoid at which the solenoid is attached to the hydraulic control valve body is provided with upper and lower through-holes communicating with a plunger compartment accommodating the plunger. The uppermost part of the upper through-hole is above or level with the uppermost part of the plunger compartment. The lowermost part of the lower through-hole is below or level with the lowermost part of the plunger compartment. The hydraulic control valve body has upper and lower vertical holes. The upper vertical hole is provided at a position above the upper through-hole provided in the cover of the solenoid in communication with the upper through-hole. The lower vertical hole is provided at a position below the lower through-hole provided in the cover of the solenoid in communication with the lower through-hole. The upper vertical hole is in communication with a tank port.
As stated above, the cover of the solenoid is provided with upper and lower through-holes extending through the cover to the plunger compartment. The uppermost part of the upper through-hole is above or level with the uppermost part of the plunger compartment. The lowermost part of the lower through-hole is below or level with the lowermost part of the plunger compartment. Thus, the air in the solenoid can be exhausted to the outside. Therefore, it is possible to stabilize the operation of the plunger of the solenoid and the operation of the spool of the hydraulic control valve.
Further, because wear particles generated in the solenoid are discharged through the lower through-hole, it is possible to prevent the plunger from sliding in wear particles, which would otherwise accelerate wear, and hence possible to improve durability.
When water is used as a working fluid for the hydraulic control valve, because the air in the solenoid is exhausted, it is possible to prevent oxidation of portions of constituent members that are in contact with water.
Because a vertical hole is provided in the hydraulic control valve body at a position above the upper through-hole in the cover of the solenoid in communication with the tank port, the air in the solenoid can be exhausted to the outside of the hydraulic control valve.
Because a vertical hole is provided in the hydraulic control valve body at a position below the lower through-hole in the cover of the solenoid, wear particles generated by the sliding movement of the plunger can be accumulated in the vertical hole. Moreover, there is no possibility that the wear particles accumulated in the vertical hole may be scattered or caused to flow backward by the operation of the hydraulic control valve.